The present invention relates to a liquid crystal display device (hereinafter referred to as LCD), and particularly to an LCD capable of displaying a high quality image by preventing non-uniformity in its display due to distortion of its transparent substrate caused by heat from a line light source of its backlight. High definition LCDs for use in notebook computers and computer monitors are such that a liquid crystal panel, a driving circuit board therefor, a light source for illuminating the panel from behind back (a so-called backlight) are stacked and fixed between an upper frame having a display window and a lower frame to form a liquid crystal display module, and this module is housed in a display part of the notebook computers and computer monitors.
The LCD of this type includes a liquid crystal panel basically formed of a pair of transparent substrates at least one of which is made of material such as glass and a liquid crystal layer sandwiched therebetween, and a driving circuit which is mounted on the periphery of the panel for applying appropriate voltages to pixel electrodes on one or both of the transparent substrates to display an image.
Methods of producing images are classified into two types:
(1) A type in which desired pixel electrodes of the liquid crystal panel are selectively switched into either their on or off states by applying voltages thereto directly and selectively; and
(2) A type in which each pixel electrode is provided with its own active element and each pixel is switched into either their on or off states by selecting its own active element.
The latter type is called an active matrix type, and is dominant over other types in terms of contrast ratio and fast optical switching speed. The conventional active matrix type LCD employs a so-called vertical field type which applies an electric field between opposing electrodes formed on a pair of substrates, respectively, for changing orientation of liquid crystal molecules therebetween. This type of LCDs are disclosed in Japanese Patent Laid-Open No. Sho 61-214548 and Japanese Utility Model Laid-Open No. Hei 2-13765, for example.
Recently, a so-called horizontal field type (also referred to as in-plane switching type, hereinafter IPS type) LCD was realized which applies to its liquid crystal layer an electric field substantially in parallel with the plane of its substrates. As for the IPS type LCD, LCDs employing both pixel electrodes in the form of interleaved combs on one of a pair of opposing substrates are known for the very wide viewing angles (Japanese Patent Publication No. Sho 63-21907 and U.S. Pat. No. 4,345,249).
In prior art LCDS, usually the longitudinal axis of a linear lamp constituting a backlight is disposed along a side (usually long side) of the liquid crystal panel, and as a result the longitudinal axis of the linear lamp is at 45xc2x0 and 135xc2x0 with respect to polarizing axes of a pair of polarizers disposed in front of and behind the liquid crystal panel.
FIG. 31 illustrates the arrangement of a liquid crystal panel and a linear lamp of a backlight in a prior art LCD, reference character SUB1 denotes a lower transparent substrate (hereinafter a TFT substrate) having thin film transistors (hereinafter TFTs) formed thereon, SUB2 is an upper transparent substrate (hereinafter a color filter substrate) having a color filter formed thereon, LP is a linear lamp, POL2S is a polarizing axis of a polarizer stacked on an outer surface of the color filter substrate SUB2, ORI2S is an alignment direction of surface alignment of liquid crystal molecules provided by an alignment film (also referred to as an orientation film) formed on an inner surface of the color filter substrate SUB2, POL1S is a polarizing axis of a polarizer stacked on an outer surface of the TFT substrate SUB1, ORI1S is an alignment direction of surface alignment of liquid crystal molecules provided by an alignment film formed on an inner surface of the TFT substrate SUB1.
When the linear lamp is disposed along the long side of a transparent substrate, heat from the linear lamp distorts the transparent substrate, and the distorted substrate exerts a birefringent action on light passing through the transparent substrate. FIG. 32 is a schematic for explaining the birefringent action by the transparent heated by the linear lamp. In FIG. 32, only a component of illuminating light L from a backlight (not shown) which is parallel to the polarizing axis POL1S of the polarizer POL1 is transmitted to a transparent substrate SU, and if distortion is caused in the transparent substrate SUB by heat, it produces a birefringent action in the transparent substrate SU, which in turn causes extraordinary rays LN to emerge from the transparent substrate SUB as well as the ordinary rays LO having passed through the polarizer POL1.
When the polarizing axis POL2S of the polarizer POL2 is perpendicular to the polarizing axis POL1S of the polarizer POL1, the ordinary rays LO is blocked by the polarizer POL2, but the extraordinary rays LN pass through the polarizer POL2. Consequently, if the transparent substrate is distorted, an LCD displays a locally whitish area due to leakage of light through the distorted area of the substrate when the LCD is intended to display a black scene.
FIG. 33 schematically illustrates a display image by the LCD for explaining non-uniformity in a display caused by distortion of their transparent substrates. If a linear lamp LP is disposed along the side of the liquid crystal panel PNL, thermal distortion occurs in a portion of the transparent substrates in the vicinity of the linear lamp LP. When a black scene A is displayed on the screen of the liquid crystal panel PNL, a locally whitish area B is observed due to birefringent action produced in the thermally distorted portion of the substrate as explained in connection with FIG. 32. Such non-uniformity in a display greatly degrades the quality of the display image, and adversely affects performance of the LCD.
An object of the present invention is to provide an LCD capable of displaying a high quality image by solving the problems with the prior art and preventing non-uniformity in a display caused by heat from a light source of its backlight.
Considering that thermal distortion in the transparent substrates caused by heat from a line light source of the backlight occurs in a direction perpendicular to a longitudinal axis of the line light source, adverse effects of birefringent action are the maximum on the incident light when a linearly polarized light enters the thermally distorted area with its plane of polarization at 45 degrees with respect to the direction of the thermal distortion, and they are minimized when its plane of polarization is parallel to or perpendicular to the direction of the thermal distortion. To accomplish the accomplish the object, in accordance with one embodiment of the present invention, there is provided a liquid crystal display device comprising: a liquid crystal panel including a pair of opposing transparent substrates each having an alignment film on an inner surface thereof, at least one of the transparent substrates having pixel electrodes, on the inner surface thereof, a liquid crystal layer sandwiched between the alignment films, and a pair of polarizers, one of which is disposed in front of the liquid crystal layer and another of which is disposed behind the liquid crystal layer; a driving circuit for supplying voltages to the liquid crystal panel in accordance with display signals; and a backlight having a line light source and disposed behind the liquid crystal panel, wherein each alignment direction of the alignment films is parallel with a polarizing axis of one of the pair of polarizers adjacent thereto, and the polarizing axis of the another of the pair of polarizers is perpendicular to a longitudinal axis of the line light source. In accordance with another embodiment of the present invention there is provided a liquid crystal display device including a liquid crystal panel having a pair of opposing transparent substrates each having an alignment film on an inner surface thereof, at least one of the transparent substrates having pixel electrodes on the inner surface thereof, a liquid crystal layer sandwiched between the alignment films, and a pair of polarizers, one of which is disposed in front of the liquid crystal layer and another of which is disposed behind the liquid crystal layer; a driving circuit for supplying voltages to the liquid crystal panel in, accordance with display signals; and a backlight having a line light source and disposed behind the liquid crystal panel, wherein alignment directions of the alignment films are parallel with each other, an alignment direction of one of the alignment films is parallel with a polarizing axis of one of the polarizers adjacent thereto, and the polarizing axis of the another of the pair of polarizers is perpendicular to a longitudinal axis of the line light source.